This operator passes through the value of the first input if the first input is ultimately connected. If the first input is not ultimately connected, it outputs the value of the second input. If neither input is connected, the operator outputs the value specified in the Defaults section of the operator parameters.
An input is not ultimately connected if it has no immediate link, or if it has a link that comes directly from a Subnet Inputs operator, and the parent subnetwork has no corresponding input connection, or that connection is also a Subnet Inputs operator, and so on.
Thus, if an input is directly connected to a Subnet Inputs operator, it is not considered to be ultimately connected unless the parent network provides an ultimately connected link to it.